There has previously been described in French Patent Application No. 74 30 259, corresponding to U.S. application Ser. No. 605,926 and assigned to the assignee of the present application, a system for measuring the movement of suspended or dissolved colloidal particles which can be used particularly for the treatment of water to determine the optimum dosage of coagulant required for clarification or for regulating the addition of coagulant.
In this known system the movement of the colloids is measured in the immediate neightbourhood of one of the electrodes used for applying an electric field. This electrode is the site of a cathodic transfer reaction allowing the passage of the electrolytic current which is formed between the electrodes, particularly in an aqueous solution. During the passage of the current, that is to say during the measurement, hydrogen is formed and hydroxyl ions are produced. The liberation of gaseous hydrogen and the formation of bubbles which are ordinarily produced at that time are avoided in the known system by use of a platinum cathode which allows the measurement to be made without disturbance.
However, the presence of hydroxyl ions brings about a local pH increase and produces in some instances secondary phenomena having the effect of varying the optical density in the region concerned, that is to say in the region of measurement.
For example, in water super-saturated in bicarbonate there is a rapid precipitation of alkaline earth bicarbonate which clouds the water. In certain highly colloidal waters or in certain coloured waters this alkalisation brings about a noticeable change in the optical density of the solution.
The extent of these interfering phenomena is proportional to the intensity of the measurement current, which is with respect to the water the electrolysis current. This current is itself for a given electrical field proportional to the conductivity of the water. In other words, in the case of a highly mineralised water these phenomena are significant and upset the measurement of optical density in the neighbourhood of the electrode.
It has been observed that in water of resistivity less than 1000 .OMEGA. cm, measurement of the variation in optical density in the neighbourhood of the electrode proves impossible.
Furthermore the passage of a significant electrical current in the cell produces, as in every electrophoresis cell, phenomena relating to transport of material wich are liable to upset the measurement.